From: Nathaniel Wesley Filardo <nwf@cs.jhu.edu>
Date: Tue, 27 Oct 2015 00:42:02 +0000 (-0400)
Subject: A few small cosmetic changes
X-Git-Url: https://hydra-www.ietfng.org/gitweb/?a=commitdiff_plain;h=d006c4d0c7f04579089b5f102c18b99055b7e19b;p=xv6-public

A few small cosmetic changes

Comments, fixing a panic() message, and making trap.c a little more debugger
friendly (now possible to breakpoint specifically on a trap-induced exit,
rather than the syscall)
---

diff --git a/proc.h b/proc.h
index 3b9c3ac..ac91b71 100644
--- a/proc.h
+++ b/proc.h
@@ -27,6 +27,14 @@ extern int ncpu;
 // holding those two variables in the local cpu's struct cpu.
 // This is similar to how thread-local variables are implemented
 // in thread libraries such as Linux pthreads.
+//
+// One must be very careful with cpu as we might migrate; that is, in
+// general, it is only safe to access this variable and *do* anything with
+// it while local preemption is disabled.
+//
+// proc is *observably constant* from a particular proc's perspective, on
+// the other hand (the scheduler always restores its value) and so is in
+// general safe to access without holding any locks.
 extern struct cpu *cpu asm("%gs:0");       // &cpus[cpunum()]
 extern struct proc *proc asm("%gs:4");     // cpus[cpunum()].proc
 
diff --git a/trap.c b/trap.c
index 6118f81..043c9e4 100644
--- a/trap.c
+++ b/trap.c
@@ -32,6 +32,13 @@ idtinit(void)
 }
 
 //PAGEBREAK: 41
+// This is here so you can break on it in gdb
+static void
+trap_exit()
+{
+  sti();
+  exit();
+}
 void
 trap(struct trapframe *tf)
 {
@@ -97,7 +104,7 @@ trap(struct trapframe *tf)
   // (If it is still executing in the kernel, let it keep running
   // until it gets to the regular system call return.)
   if(proc && proc->killed && (tf->cs&3) == DPL_USER)
-    exit();
+    trap_exit();
 
   // Force process to give up CPU on clock tick.
   // If interrupts were on while locks held, would need to check nlock.
@@ -106,5 +113,5 @@ trap(struct trapframe *tf)
 
   // Check if the process has been killed since we yielded
   if(proc && proc->killed && (tf->cs&3) == DPL_USER)
-    exit();
+    trap_exit();
 }
diff --git a/vm.c b/vm.c
index b340696..a58ef21 100644
--- a/vm.c
+++ b/vm.c
@@ -267,7 +267,7 @@ deallocuvm(pde_t *pgdir, uint oldsz, uint newsz)
     else if((*pte & PTE_P) != 0){
       pa = PTE_ADDR(*pte);
       if(pa == 0)
-        panic("kfree");
+        panic("deallocuvm");
       char *v = P2V(pa);
       kfree(v);
       *pte = 0;